(1) Field of the Invention
The present invention relates to an arc tube that has spiral parts wound around an axis, a discharge lamp equipped with the arc tube, and a production method of the arc tube.
(2) Related Art
In the present energy-saving era, discharge lamps, exhibiting high luminous efficiency and long life, are calling attentions as light sources alternative to incandescent lamps. The representatives of such discharge lamps are compact self-ballasted fluorescent lamp and fluorescent lamp. The compact self-ballasted fluorescent lamp (hereinafter simply called “lamp”) and the fluorescent lamp have a glass tube, as their component, whose inner surface is provided with a phosphor coating.
The phosphor coating is excited in response to irradiation of ultraviolet lights, thereby emitting visible light towards outside of the glass tube in the thickness direction of the phosphor coating. However, the same amount of visible light as that emitted outside the glass tube is also irradiated towards inside of the glass tube. This visible light emitted towards inside of the glass tube is, in turn, partly absorbed by the phosphor coating situating at the opposing side in a cross section of the glass tube. The remainder of the visible light unabsorbed is irradiated towards outside of the glass tube.
The amount of visible light irradiated towards inside of the glass tube increases as the thickness of the phosphor coating increases, and taking advantage of this feature, discharge lamps have been developed that enable the illuminance in the illumination direction to improve (e.g. Japanese Laid-open Patent Application H8-339781).
In the discharge lamp in this prior art, a glass tube constituting the arc tube has a turning part at the substantial center between the two ends of the glass tube, and is wound around an axis from this turning part to the ends, so as to form a double-spiral configuration. In addition, the phosphor coating provided on the inner surface of this glass tube is thicker near the inner side of the spiral configuration (i.e. near the axis), and thinner near the outer side of the spiral configuration. To be more specific, at a cross section of the glass tube, suppose taking two areas of the inner surface of the glass tube, that face each other in a direction that passes through the center of the glass tube and that is substantially orthogonal to the axis. Then, the phosphor coating is thicker in the area which is nearer the axis, than in the other area which is farther from the axis.
Therefore, the amount of the visible light emitted from the entire arc tube in orthogonal and opposite direction to the axis is a summation of: visible light emitted from the area farther from the axis; and visible light emitted from the area nearer the axis. As a result, the illuminance in the orthogonal direction will improve, compared to the illuminance in the other directions.
In the conventional arc tube, at a cross section of the glass tube, the thickness of its phosphor coating is more nearer the axis, and less farther from the axis. Accordingly, it is inevitable that large illuminance is obtained in orthogonal direction to the axis.
Normally, the arc tube of a lamp is used under a state mounted to a lighting device set to the ceiling in advance. In such a case, the turning part will be directed downward. Therefore, there is a problem relating to conventional arc tubes, that compared to the enhanced illuminance in the lateral direction of the arc tube, the downward direction thereof in which illuminance is required will not be illuminated so much.